Handbook of Aggregation-Induced Emission, Volume 1. Группа авторов

made the decay of emission band much longer than 18 and 19. For 22 or 23 with a rigid structure, the formation of the photocyclized intermediate took place directly on the subpicosecond timescale so that the fluorescence was very weak.

Figure 3.29 TPE derivatives 18–23 with increased structural rigidity and their transformation upon UV irradiation (QY: fluorescence quantum yield).

      Source: Reproduced with permission from Ref. [57]. Copyright 2018, Royal Society of Chemistry.

There is a competitive relationship between the two processes of photocyclization and intramolecular rotation. When TPE possessed a flexible structure, the rotations of phenyl rings prohibited the photocyclization between two adjacent phenyl rings and allowed the excited double bond to rotate. But when phenyl rings are hinged by ethylene bridge, the short distance of rings promoted the ultrafast formation of the photocyclization on the subpicosecond timescale, giving no opportunity for the C═C bond to rotate. Just like molecule 20, only after these two nonradiative processes were blocked at the same time, TPE derivatives could render strong emission.

Figure 3.30 The PES of 18 in the ground state and excited state as a function of the (quasi) C═C bond twisting and phenyl torsion dihedral angles. (a) Top view of the first excited‐state PES. (b) Top view of the ground‐state PES.

      Source: Reproduced with permission from Ref. [57]. Copyright 2018, Royal Society of Chemistry.

In 2018, Sada et al. [58] disclosed the RDBR process of disubstituted TPE derivatives TPE‐2OMe and TPE‐2F through a combination of photochemical experiments and theoretical computations. As shown in Figure 3.31, E‐ or Z‐rich isomers exhibited EZI behavior after the solution was exposed to UV irradiation. Photostationary state approached in four hours under a deep‐ultraviolet (deep‐UV) lamp irradiation (6.2 mW/cm²) or in 48 hours under ambient light. Furthermore, the solution in dark conditions or the solid under a deep‐UV lamp did not show EZI phenomenon, revealing that the EZI process was triggered by UV irradiation and was suppressed in the aggregated state. However, no photocyclization was observed in ¹H NMR measurements, indicating that isomerization was indeed contained in the fluorescence measurement process, rather than the photocyclization.

The more detailed process was simulated by calculating the steepest‐descent (SD) pathways in the S₁ states for E‐ or Z‐isomer, starting from the FC structures. Along the SD pathways, the rotational motion around the central double bond leads to the perpendicular structure. As the change of the TPE structure, the S₁ energy gradually decreased and the S₀ energy increased accordingly. Eventually, the S₁ and S₀ energies came to the closest when the double bond twisted about 90°, suggesting the existence of a CI near that place (see Figure 3.32).

Figure 3.31 (a−c) Photoisomerization of TPE‐2OMe and TPE‐2F in chloroform (a) under deep‐UV lamp irradiation, (b) under ambient‐light irradiation, and (c) in the dark. (d) Photoisomerization of TPE‐2OMe and TPE‐2F in the solid state.

      Source: Reproduced with permission from Ref. [58]. Copyright 2017, American Chemical Society.

      In addition to free TPE‐2OMe monomer, the behavior in the crystal state was also simulated. From the crystal computational results, the torsion of the double bond was strictly inhibited by the other surrounding molecules, leading to only an 8° change of twisting angle. However, the twist of phenyl rings in the crystal state was identical to that of monomer in the excited state because the dihedral angle of the phenyl ring showed a similar variation (63° at S_0min → 45° at S_1min). This revealed that the double bond rotation triggered by photoirradiation rather than the phenyl ring rotation played a key role on the AIE effect.

      3.2.5 Other AIEgens Involving RBDR Process

      In addition to TPE, there are many other AIEgens with a double bond, in which the RDBR process is also involved in their luminescence emission.

Figure 3.32 Energy variations of the S₁ and S₀ states along the steepest‐descent (SD) pathway in the S₁ state of TPE‐2OMe.

      Source: Reproduced with permission from Ref. [58]. Copyright 2017, American Chemical Society.

Figure 3.33 Molecular structures of dinitriles DCNT and DCNP.

Kobayashi et al. [59] prepared dinitriles DCNT and DCNP (see Figure 3.33) that exhibited AIE and isomerization properties. When the solution of their E‐ or Z‐isomer was exposed to a UV lamp, the central ethylenic bond substituted by cyano groups could rotate and result in photoisomerization and fluorescence quenching. In the packed state, no isomerization of them was observed on the same experimental conditions due to the locked conformation of the compounds, providing a bright emission.

The (E)‐CN‐MBE is a typical AIEgen having great photophysical and self‐assembling characteristics, whose Φ_f is dramatically enhanced almost 700‐fold from solution to aggregation [8]. But (Z)‐CN‐MBE was the opposite, which emitted no fluorescence in both solution and aggregated states. Park et al. reported that the solid (Z)‐CN‐MBE became intense emissive when it was exposed to a UV lamp under ambient temperatures due to the EZI process [60]. It was thought that the bent‐shape structure of (Z)‐CN‐MBE led to loose packing, which was unable to effectively restrict the rotation of double bond even in the solid state. Therefore, nonradiative photoisomerization occurred. In contrary, the planar molecular structure of (E)‐CN‐MBE was easier to form tight packing, effectively blocking the double bond rotation (see Figure 3.34).

      This inference was confirmed by Yamamoto’s calculation results [61]. Electronic structural calculations were employed to analyze the mechanisms of AIE and photo/thermal E/Z isomerization of CN‐MBE. In addition